“There is certainly much more good news than bad news, in terms of coffee and health,” says Frank Hu, MD, MPH, PhD, nutrition and epidemiology professor at the Harvard School of Public Health.

But (you knew there would be a “but,” didn’t you?) coffee isn't proven to prevent those conditions.

Researchers don't ask people to drink or skip coffee for the sake of science. Instead, they ask them about their coffee habits. Those studies can't show cause and effect. It's possible that coffee drinkers have other advantages, such as better diets, more exercise, or protective genes.

So there isn't solid proof. But there are signs of potential health perks -- and a few cautions.

If you're like the average American, who downed 416 8-ounce cups of coffee in 2009 (by the World Resources Institute's estimates), you might want to know what all that java is doing for you, or to you.

Here is a condition-by-condition look at the research.

Type 2 Diabetes

Hu calls the data on coffee and type 2 diabetes "pretty solid," based on more than 15 published studies.

"The vast majority of those studies have shown a benefit of coffee on the prevention of diabetes. And now there is also evidence that decaffeinated coffee may have the same benefit as regular coffee,” Hu tells WebMD.

In 2005, Hu's team reviewed nine studies on coffee and type 2 diabetes. Of more than 193,000 people, those who said they drank more than six or seven cups daily were 35% less likely to have type 2 diabetes than people who drank fewer than two cups daily. There was a smaller perk -- a 28% lower risk -- for people who drank 4-6 cups a day. The findings held regardless of sex, weight, or geographic location (U.S. or Europe).

Continued

More recently, Australian researchers looked at 18 studies of nearly 458,000 people. They found a 7% drop in the odds of having type 2 diabetes for every additional cup of coffee drunk daily. There were similar risk reductions for decaf coffee drinkers and tea drinkers. But the researchers cautioned that data from some of the smaller studies they reviewed may be less reliable. So it's possible that they overestimated the strength of the link between heavy coffee drinking and diabetes.

How might coffee keep diabetes at bay?

“It’s the whole package,” Hu says. He points to antioxidants -- nutrients that help prevent tissue damage caused by molecules called oxygen-free radicals. “We know that coffee has a very strong antioxidant capacity," Hu says.

It's probably not the caffeine, though. Based on studies of decaf coffee, “I think we can safely say that the benefits are not likely to be due to caffeine," Hu says.

Hold the Caffeine?

The fact that coffee contains good stuff does not necessarily mean that it’s good for us, says James D. Lane, PhD, professor of medical psychology and behavioral medicine at Duke University Medical Center in Durham, N.C.

“It has not really been shown that coffee drinking leads to an increase in antioxidants in the body,” Lane tells WebMD. “We know that there are antioxidants in large quantities in coffee itself, especially when it’s freshly brewed, but we don’t know whether those antioxidants appear in the bloodstream and in the body when the person drinks it. Those studies have not been done.”

Regular coffee, of course, also contains caffeine. Caffeine can raise blood pressure, as well as blood levels of the fight-or-flight chemical epinephrine (also called adrenaline), Lane says.

Heart Disease and Stroke

Continued

Besides that, coffee has been linked to lower risks for heart rhythm disturbances (another heart attack and stroke risk factor) in men and women, and lower risk for strokes in women.

In a study of about 130,000 Kaiser Permanente health plan members, people who reported drinking 1-3 cups of coffee per day were 20% less likely to be hospitalized for abnormal heart rhythms (arrhythmias) than nondrinkers, regardless of other risk factors.

And, for women, coffee may mean a lower risk of stroke.

In 2009, a study of 83,700 nurses enrolled in the long-term Nurses' Health Study showed a 20% lower risk of stroke in those who reported drinking two or more cups of coffee daily compared to women who drank less coffee or none at all. That pattern held regardless of whether the women had high blood pressure, high cholesterol levels, and type 2 diabetes.

Parkinson’s and Alzheimer's Diseases

“For Parkinson’s disease, the data have always been very consistent: higher consumption of coffee is associated with decreased risk of Parkinson’s,” Hu tells WebMD. That seems to be due to caffeine, though exactly how that works isn't clear, Hu notes.

Coffee has also been linked to lower risk of dementia, including Alzheimer’s disease. A 2009 study from Finland and Sweden showed that, out of 1,400 people followed for about 20 years, those who reported drinking 3-5 cups of coffee daily were 65% less likely to develop dementia and Alzheimer’s disease, compared with nondrinkers or occasional coffee drinkers.

Cancer

The evidence of a cancer protection effect of coffee is weaker than that for type 2 diabetes. But “for liver cancer, I think that the data are very consistent,” Hu says.

“All of the studies have shown that high coffee consumption is associated with decreased risk of livercirrhosis and liver cancer,” he says. That's a "very interesting finding," Hu says, but again, it's not clear how it might work.

Again, this research shows a possible association, but like most studies on coffee and health, does not show cause and effect.

Continued

Pregnancy

In August 2010, the American College of Obstetricians and Gynecologists (ACOG) stated that moderate caffeine drinking -- less than 200 mg per day, or about the amount in 12 ounces of coffee -- doesn't appear to have any major effects on causing miscarriage, premature delivery, or fetal growth.

But the effects of larger caffeine doses are unknown, and other research shows that pregnant women who drink many cups of coffee daily may be at greater risk for miscarriage than non-drinkers or moderate drinkers. Again, it's not clear whether the coffee was responsible for that.

Calories, Heartburn, and Urine

You won't break your calorie budget on coffee -- until you start adding the trimmings.

According to the web site myfoodapedia.gov -- part of the U.S. Department of Agriculture's Center for Nutrition Policy and Promotion -- a 6-ounce cup of black coffee contains just 7 calories. Add some half & half and you'll get 46 calories. If you favor a liquid nondairy creamer, that will set you back 48 calories. A teaspoon of sugar will add about 23 calories.

Drink a lot of coffee and you may head to the bathroom more often. Caffeine is a mild diuretic -- that is, it makes you urinate more than you would without it. Decaffeinated coffee has about the same effect on urine production as water.

Both regular and decaffeinated coffee contain acids that can make heartburn worse.